Press Release No. 925

For use of the information media Not an official record

WMO Issues new El Niño/La Niña Update

Neutral Conditions or Weak La Niña most likely scenarios for rest of 2011

Geneva, 1 September 2011 (WMO) – The World Meteorological Organization (WMO) has issued its latest Update on El Niño/La Niña, the phenomena in the tropical Pacific which have important consequences upon weather and climate around the globe.

The continuation of near neutral conditions - with the ocean temperatures, tropical rainfall patterns, and atmospheric winds over the equatorial Pacific Ocean near the long-term average – or the re-emergence of La Niña conditions are the most likely scenarios for the rest of 2011, according model forecasts and expert interpretation used by WMO.

If a La Niña event does occur, current indications are that it would be considerably weaker than the moderate to strong 2010-2011 episode, which ended in May 2011 and was linked to disastrously wet conditions in parts of Australia, Indonesia, Southeast Asia, and portions of northern South America (e.g. Colombia), and drought in parts of the Horn of Africa, central southwest Asia, southeastern South America and the southern United States of America.

Development of an El Niño is considered very unlikely, the Update said.

El Niño is characterized by unusually warm ocean surface temperatures in the central and eastern tropical Pacific. La Niña is the opposite and is characterized by unusually cool ocean surface temperatures in the same area. Both events are strongly coupled to the atmospheric circulation in the tropics, and are known to significantly influence weather and climate patterns around the globe. However, there are also other factors which influence seasonal climatic patterns in different parts of the world, which need to be considered along with information on El Niño/La Niña.

Following the dissipation of a moderate to strong La Niña event that dominated the tropical Pacific Ocean from September 2010 to February 2011, neutral conditions prevailed over the tropical Pacific from May 2011 onwards. However, a few weak remnants of La Niña persisted, particularly in some atmospheric features.

Observations during recent weeks indicate a drift toward the cool side of neutral in terms of surface as well as subsurface ocean temperatures, along with associated changes in atmospheric circulation.

“Together, these changes indicate a shift in the odds of a weak La Nina forming during the forthcoming months. Overall the complete set of forecast models indicates a range of possible outcomes for the remainder of 2011. Their consensus continues to favour near-neutral conditions, but only by a small margin over weak La Niña conditions,” the Update said.

“Given the current uncertainty, close monitoring is required for any firmer signs of possible future developments,” it said.

The El Niño/La Niña Update is a consensus-based product prepared by WMO in close collaboration with the International Research Institute for Climate and Society (IRI), USA, based on input from climate prediction centres and experts around the world.

El Niño – Spanish for boy child because it often develops in December - and La Niña represents opposite extremes in the ENSO cycle.

El Niño refers to the above-average sea-surface temperatures that periodically develop across the east-central equatorial Pacific. It represents the warm phase of the ENSO cycle.

La Niña refers to the periodic cooling of sea-surface temperatures across the east-central equatorial Pacific. It represents the cold phase of the ENSO cycle.

ENSO-neutral refers to those periods when neither El Niño nor La Niña is present. These periods often coincide with the transition between El Niño and La Niña events. During ENSO-neutral periods the ocean temperatures, tropical rainfall patterns, and atmospheric winds over the Equatorial Pacific Ocean are near the long-term average.

The fluctuations in ocean temperatures during El Niño and La Niña are accompanied by even larger-scale fluctuations in air pressure between the western and eastern tropical Pacific known as the Southern Oscillation.

Generally, during La Niña episodes rainfall is increased across the western equatorial Pacific, including northern Australia and Indonesia during December-February and the Philippines during June-August and is nearly absent across the eastern equatorial Pacific. Wetter than normal conditions also tend to be observed during December-February over northern South America and southern Africa, and during June-August over South Asia and southeastern Australia. Drier than normal conditions are generally observed along coastal Ecuador, northwestern Peru and equatorial eastern Africa during December to February, and over southern Brazil and central Argentina during June-August.

La Niña is also known to be associated with a relatively more active hurricane season in tropical North Atlantic, during June to November.

ENSO being in a neutral state does not imply that climate patterns typically associated with it would be close to normal; there can be several other influences on regional and local climate, which need to be adequately taken into account.